The present invention generally relates to tube suspension systems for magnets and, more particularly, is concerned with a laminated composite shell assembly having joint bonds.
Superconductive magnets include superconductive coils which generate uniform and high strength magnetic fields, such as used, without limitation, in magnetic resonance imaging (MRI) systems employed in the field of medical diagnostics. The superconductive coils of the magnet typically are enclosed in a cryogenic vessel surrounded by a vacuum enclosure and insulated by a thermal shield interposed therebetween.
Various designs of tube suspensions are employed to support the cryogenic vessel of a superconductive coil assembly of the magnet from and in spaced apart relation to both the thermal shield and the vacuum enclosure of the magnet. As one example, the tube suspension can include overlapped fiberglass outer and inner support cylinders, such as disclosed in U.S. Pat. No. 5,530,413 to Minas et al. which is assigned to the same assignee as the present invention. In the Minas et al. tube suspension, the outer support cylinder is located within and generally spaced apart from the vacuum enclosure and positioned outside of and generally spaced apart from the thermal shield. A first end of the outer support cylinder is rigidly connected to the vacuum enclosure while a second end of the outer support cylinder is rigidly connected to the thermal shield. The inner support cylinder is located within and generally spaced apart from the thermal shield and is positioned outside of and generally spaced apart from the superconductive coil assembly. The inner support cylinder has a first end rigidly connected to the thermal shield near the second end of the outer support cylinder and has a second end located longitudinally between the first and second ends of the outer support cylinder and rigidly connected to the superconductive coil assembly.
Problems can occur, however, with some designs of tube suspension systems at cryogenic temperatures. For instance, tube suspensions of some current superconductive magnet designs in MRI systems employ metal alloys or glass-epoxy materials. Metal alloys as well as glass-epoxy materials do not provide optimal load distributing and thermal insulating characteristics. Further, metal alloys are heavy and glass-epoxy materials deform as they tend to be compliant. Also, adhesive bonded joints are oftentimes supported by bolts or rivets. Such joints tend to act as a local weak link in the assembled structure. In the presence of cryogenic environments and combined thermo-mechanical loads, adhesive bonds become stiffer and can result in regions of localized failure leading to a compromise in structural integrity.
Consequently, a need exists for innovation with respect to tube suspensions for supporting superconductive magnets which will provide a solution to the aforementioned problems.
The laminated composite shell assembly of the present invention employs one or more laminated pinch rings reinforcing one or more joint bonds of the assembly. The laminated composite shell assembly per se has a sequence of laminated composite shells which comprises the invention of patent application Ser. No. 09/196,423 assigned to the assignee of the present invention. The laminated composite shells are made of composite layers having fibers (such as graphite-epoxy material) and assembled together to form the assembly. The graphite-epoxy material is stiffer than glass-epoxy material and tends to deform elastically rather than plastically. The use of bolts or rivets to support joint bonds connecting the assembled composite shells to external structures would cause the holes in the laminates to act as stress concentrators and weaken the assembly. The introduction of laminated pinch rings adjacent the locations of the joint bonds in place of bolts or rivets eliminates the need for holes in the laminates and so avoids the problem of stress concentration. The laminated pinch rings reinforce the joint bonds by providing additional stiffness and simultaneously acting as load redistribution members.
In an embodiment of the present invention, a laminated composite shell assembly is provided which can be used in a tube suspension for a magnet. The laminated composite shell assembly includes a plurality of laminated composite shells assembled to one another and having one or more joint bonds at which the composite shells are connected to an external structure. The assembly further includes one or more laminated pinch rings assembled to the composite shells adjacent to the joint bonds so as to reinforce the joint bonds.
The composite shells and pinch rings are substantially cylindrical in configuration and have a predetermined axial direction. Each shell and pinch ring is made of composite layers having fibers with the fibers being oriented in a plurality of stacking sequences with reference to the axial direction of the shell and pinch ring. An example of the fibers of the shells and pinch rings are graphite fibers of a graphite-epoxy material. The shells are concentrically assembled in a desired sequence with some of the shells being adapted to perform a structural load bearing function while others of the shells are adapted to perform a load transfer function. The laminated pinch rings are concentrically assembled to the shells so as to perform additional stiffening as well as load redistributing and transfer functions in the regions of the joint bonds between the assembled shells and the external structure.